Enzymes are protein molecules that are able to catalyse a biological reaction.

 Each type of enzyme is highly specific for only one type of a reaction  With the use of an enzyme the rate of a particular reaction can increase by a factor greater than 10 8  They work by providing an alternative pathway with a lower activation energy  More reactant molecules then have the necessary minimum activation energy

 The specificity of the enzyme depends on their tertiary and quaternary structure.  The substrate (reactant molecule) binds to a part of the enzyme known as the active site  The active site is able to change shape allowing the substrate to fit effectively and this is known as the induced fit theory

 Inhibitors are substances that slow down the rate of an enzyme catalysed reaction  Competitive inhibitors resemble the substrate in shape but they are unable to react  They occupy the active site making it less accessible to the substrate  As the concentration of the substrate molecules increases the effect of the competitive inhibitors decreases

Competitive inhibition

 Non-competitive inhibitors bind to the enzyme but not to the active site  The enzyme then changes shape and the substrates are no longer able to bind  With this type of inhibitor increasing the concentration of the substrate molecules will have no effect as the non-competitive inhibitors don’t bind to the active site

Non-competitive inhibition

 At low substrate concentrations the rate of the reaction is proportional to the concentration of the substrate  At high substrate concentrations the rate of the reaction reaches a maximum point known as Vmax  This means that that at low substrate concentrations there are enough active sites for the substrates to bind and react  When all the active sites are used the enzyme is not able to work any faster

 This constant Km is the concentration of the substrate at half of the Vmax  It is always the same for a particular enzyme with a particular substrate  It indicates whether the enzyme functions appropriately at low substrate concentrations or whether high substrate concentrations are needed for the efficient catalysis  Lower Km → more efficient enzyme

Competitive vs Non-competitive  Competitive inhibitor  Blocks active sites  Same Vmax  Larger Km  Non-competitive  Enzyme changes shape  Smaller Vmax  Same Km

 An increase in temperature at first increases the rate of the reaction until the optimum temperature is reached (usually 40°C)  Until then more reactants posses the necessary activation energy to react  After the optimum temperature is reached the enzyme starts to denaturate as the bonds holding the structure together break  At certain pH values there is a change in the charge of the amino acid which affects the bond and alters the structure of the enzyme making it inactive

 Heavy metals “poison” the enzyme  They react with the –SH group and replace the hydrogen atom with a heavy metal atom, or ion altering once more the structure of the enzyme

Effect of temperature and pH on rate of enzyme activity